Secondary Instability of Streamwise Vortices in a Turbulent Convex Wall Jet

The outer part of a wall jet flowing over a convex wall satisfies the centrifugal instability criterion. Such a configuration has been studied in this work with a wall jet of air flowing over the exterior surface of a 0.91-m long circular cylinder. This flow scenario has found practical use in NOTAR helicopters for side force generation and in airfoil trailing edge designs for lift augmentation. As the convex wall jet evolves, centrifugal instability manifests itself in the form of unsteady streamwise vortices whose spanwise wavelength scales with wall jet thickness. These vortices enhance mixing between the wall jet and ambient flow and can be used as a control mechanism through appropriate use of secondary instability affecting them. In this work, micro-vortex generators are positioned at the wall jet nozzle lip in order to force stronger disturbances and preset the wavelength. Planar, stereo and TR-PIV have been conducted to characterize the evolution of the vortices. The coherent streamwise vortices undergo breakdown into non-coherent turbulence at a downstream location that depends on the preset vortex wavelength. Various decomposition techniques such as spatial FFT, POD and DMD have been used to study the modes of secondary instability leading to the breakdown process.